M. Requin

I have just finished some work on my 86.5 auto 928 that has had it sitting on my lift since mid-April. It started up fine, but on the first test ride it felt like it was running on four cylinders under load. It starts readily and idles easily, and revs pretty normally when not under load- maybe a slight hesitation when the accellerator is first pushed. Here's the whole sequence of events:

1. The car had been lightly "stuttering" under sharp acceleration, typical of a shorting plug wire or similar ignition fault.

2. It went up on the lift to get new motor mounts, oil pan gasket, studs and drain valve, and new stabilizer bar bushings. I also got new Beru wires since I knew I needed them and thought they might cure the stuttering problem.

3. When I finished the work underneath the car and took it off the lift, I also took the MAF out and cleaned it (CRC MAF cleaner) since I had loosened the air box before raising the engine. I also replace the DS coil wire with a new Beru wire (using old caps that looked perfectly clean- I had replaced the coil end a few years ago).

So those are all the things I touched. Opinions on where to start? I have a hunch but need your input!! My DD is undrive-able until I get this fixed.

Mrmerlin

did you replace the passenger side coil wire?
NOTE on the coil wires you want to have the coil wires not touching any part of their run
How old is the MAF?
Is the temp 2 sensor connector in good condition?
Did you replace the TPS short harness?
How old is the O2 sensor

davek9

Get a cheep Timing light fro HF and use it to see if both "Ignition" banks are firing, starting w/ the wires that crossover in the front of the engine.

If both coils are working, good, however you will need to see how strong of a spark each bank is producing, re-check all wires, on AO's Coke car we found a broken/cracked distributor coil connection and a bad coil wire too.

If you need to start pulling plugs to see which bank's not working.

The "stuttering" on acceleration is either weak Spark or incorrect AFR due to a a bad MAF or an intake leak.

dr bob

Make sure the new coil wire is actually seating and connected. Confirm firing order with the new wires. Dave's suggestion on using the timing light as a spark-energy detector is good. All good advice.

Generally, I start troubleshooting by working backwards through work I just did. In your case, I'd add a spark plug inspection after the engine has been run for just a few minutes at idle. The non-sparking plugs will still have a raw fuel fragrance to them. Let it warm up and you may lose that telltale. Work backwards from the suspect plugs towards the coil.

M. Requin

Did not replace the PS coil wire, will check this evening. (And absolutely touching nothing- too much MC experience...)
The MAF is probably original, 86 and 200K miles.
I don't know about the temp 2 sensor or the O2 sensor, and did not replace the TPS short harness. All are likely original.

Thanks for this, I am going to try to take the input from the forum and plan/prioritize my diagnostic path. Looking at past threads on the MAF, it looks like the common failure mode means bad idle, which seems to me the sensors are a greater priority.

One thing I think is important is that this condition did not exist before I took the car off the road for work, and it has sat for 2 months.

Thanks, Stan - I'll likely be getting back to you!

M. Requin

I'll double-check the wire connections, there is room for error here. As I replied to Dave. I have done the timing light check. Firing order is not an issue since those weren't "touched".

Good advice, and many thanks!

M. Requin

Gentlemen, thanks, my worry here is not in ultimately finding the problem, but wasting time chasing my tail while I do so. Input from the forum has now (and I hope will continue to) helped me do a step-by-step plan of attack.

More comments will be most useful!

Mrmerlin

Well since your car has 200K miles you will need to
Swap in a new pass coil wire.

Get a fresh MAF, send it out for rebuild,

replace the temp 2 connector and sensor ,

Replace the O 2 sensor with a factory part,

swap out the TPS short harness,
you will probably find it broken where it plugs into the car side of the harness.

If the ISV has more than 75 K miles then replace it.

These parts must all be in top condition for the engine to run as it was designed,

NOTE any one not perfect will cause running issues

M. Requin

Ah Stan, sadly, all true! Immediate question is why the sudden change while the car was on the lift- more for curiosity's sake than anything else, of course. I want to buy some stock in Roger's enterprise, BTW...

Mrmerlin

who knows maybe you pulled the cats far enough and the O2 sensor wires shorted out.
maybe the MAF plug has shedding insulation inside the boot.
Maybe the TPS short cable connector has failed from lifting the engine.
Maybe one or both of the coil wires got damaged.
Maybe you pinched the rear cylinder harness into the firewall when lifting the engine,
or damaged a damper vacuum port..;
Fitting all new parts should by most accounts rule them out as a running issue.
Thus its usually easier to find and fix the cause

M. Requin

Grist for the mill! Thanks, exactly what I need to look at (along with your other suggestions). She'll be flying again, and soon, I promise. (I'm putting up another thread about recent work just to show a couple of ideas I came up with to make solo work easier, so you can see I have been fairly diligent in my repairs.)

M. Requin

Two distributor plug wire caps were corroded, cleaned and refitted, car runs like a top. Doubtless a consequence of sitting in a hot , humid environment for two months.